In the physiology of visual receptor cells, the processes intervening between the absorption of photons by a visual pigment and the change in membrane voltage (the receptor potential) are unknown. In general, receptor potentials are thought to be generated by changes in the ionic permeabilities of the receptor cell membrane. Metabolically driven "pumps" maintain the internal concentrations of the participating ions. The proposed research is directed at the elucidation of the detailed ionic bases of the mechanisms generating receptor potentials and the mechanisms intervening between photopigment chemistry and the receptor potential. In particular, the depolarizing receptor potential of the photoreceptors in ventral eye of Limulus, and both depolarizing and hyperpolarizing receptor potentials of the median ocellus of Limulus will be studied.